For many years, the design of concrete structures imitated typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive (vertical) load, is extremely weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile (horizontal) forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced-concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as 100 feet can be attained in members as deep as three feet for roof loads. The basic principal is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principal, but the reinforcing is held loosely in place while the concrete is placed around it. The reinforcing is then stretched by hydraulic jacks and securely anchored into place. Prestressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly in such post-tensioning operations, there is provided a pair of anchors for anchoring the ends of the tendons suspended therebetween. In the course of installing the tendon tensioning anchor assembly in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of the tendon for applying a predetermined amount of tension to the tendon. When the desired amount of tension is applied to the tendon, wedges, threaded nuts, or the like, are used to capture the tendon and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
Metallic components within concrete structures may be come exposed to many corrosive elements, such as de-icing chemicals, sea water, brackish water, or spray from these sources, as well as salt water. If this occurs, and the exposed portions of the anchor suffer corrosion, then the anchor may become weakened due to this corrosion. The deterioration of the anchor can cause the tendons to slip, thereby losing the compressive effects on the structure, or the anchor can fracture. In addition, the large volume of by-products from the corrosive reaction is often sufficient to fracture the surrounding structure. These elements and problems can be sufficient so as to cause a premature failure of the post-tensioning system and a deterioration of the structure.
FIGS. 1 and 2 illustrate various components of a typical post-tension assembly designated generally at 10. System 10 includes a tendon 12 having an exposed end protruding from a sheath 14. The end of the tendon 12 is typically fitted through an extension tube 16. Extension tube 16 has a diameter slightly larger than sheath 14 such that one end 16a of tube 16 may overlie sheath 14. The opposite end 16b of tube 16 fits over, and communicates with, a rear tubular portion 18 of an anchor 20. Rear tubular member 18 includes an aperture (not shown) which communicates with a frontal aperture 22. Frontal aperture 22 defines a cavity in which wedges 24 and 26 are received as shown in FIG. 2, below.
FIG. 2 illustrates an assembled view (in one-fourth cutaway perspective) of system 10 shown in FIG. 1. As known in the art, tendon 12 is disposed through extension tube 16 and through anchor 20. In one known embodiment, end 16b of extension tube 16 is force-fitted over rear tubular member 18. The other end 16a of extension tube 16 is sealed to sheath 14, by use of tape or other means.
After tendon 18 extends through frontal aperture 22 (see FIG. 1), and assuming the far end of the tendon (not shown) is fixed in place, tension is applied to tendon 16, typically by use of a hydraulic jack. While applying this tension, wedges 24 and 26 are forced in place on both sides of tendon 12 within the wedge cavity defined by aperture 22. Once in place, teeth 24a and 26a of wedges 24 and 26 operate to lock tendon 12 in a fixed position with respect to anchor 20. Thereafter, the tension supplied by the hydraulic device is released and the excess tendon extending outward from anchor 20 is cut by a torch or other known device. Wedges 24 and 26 thereafter prevent tendon 12 from releasing its tension and retracting inward with respect to anchor 20. Moreover, this tension provides additional tensile strength across the concrete structure.
After years of work with the anchor body of the prior art, it was found that the cavity used in the anchor body created many problems. The cavity in the anchor body is of a constantly diminishing diameter extending from a forward end of the anchor body to a rearward end of the anchor body. This internal cavity of constantly diminishing diameter is formed during the casting of the anchor body. Unfortunately, the narrow diameter end of the cavity creates problems with the installation of tendons in a corrosion-resistant environment.
When the anchor body is used in the formation of intermediate anchorages, it is often necessary to move the anchor body over a very long length of sheathed tendon. If there is insufficient clearance between the narrow diameter end of the cavity and the outer diameter of the sheathed portion of the tendon, nicks, abrasions, and cuts can occur in the corrosion-resistant sheathing. As such, the integrity of the anchorage system is impaired. Furthermore, there are circumstances where the sheathing may exceed expected tolerances and will prevent the anchor body from easily sliding along the length of the tendon so as to assume its position as an intermediate anchorage. Additionally, in recent years, there has been a tendency to increase the thickness of the sheathing so as to facilitate greater protection of the tendon from corrosive elements.
An easy solution to this problem would be to expand the diameter of the cavity so as to avoid the aforementioned problems. Unfortunately, if the diameter of the cavity is expanded, then conventional wedges cannot be used. Problems would further occur because of the use of larger wedges or of irregular wedges. If the cavity were enlarged, then the wedge components would have to be replaced in all such post-tension anchor systems. Furthermore, the use of variant sized wedges could create new problems associated with the tensioning of the anchor system.
It is also possible to drill out the narrow diameter end of the cavity so as to produce a portion of generally constant diameter. However, any past attempts at drilling have been unsuccessful for a number of reasons. First, the drilling is a very expensive process in comparison with the casting of the anchors. Furthermore, the drilling of a constant diameter portion in the anchor body can create burrs and deformations which could potentially cut the sheathing of the tendon and cause adverse corrosion-protection results. Finally, the drilling of the hole can intrude into the wedge-receiving area so as to create an uneven and irregular contact area between the wedges and the wall of the cavity.
U.S. patent application Ser. No. 09/007,608, filed on Jan. 15, 1998, by the present inventor, describes an improved anchor for a post-tension system which has a cavity with a first portion of constantly diminishing diameter and a second portion of constant diameter. The first and second portions are coaxial and communicate with each other. The first portion extends inwardly from one end of the anchor body while the second portion extends inwardly from the opposite end of the anchor body. This improved anchor is a cast anchor. However, it was found that the formation of the second portion of "constant diameter" created problems during the casting process. It is known that for making cast objects, it is often difficult to cast or form cavities of constant diameter. Constant diameter cavities in objects often require complex forms and molds in order to create such constant diameter cavities. As such, the constant diameter second portion created certain manufacturing problems.
It is an object of the present invention to provide an improved anchor for a post-tension anchor system which allows for the use of existing wedges in the wedge cavity while enlarging the narrow end of the cavity.
It is a further object of the present invention to provide an improved anchor body with a cavity with a wide end which requires no machining.
It is a further object of the present invention to provide an anchor body that avoids sharp edges and irregular contact surfaces.
It is still another object of the present invention to provide an anchor body which enhances the corrosion resistance of the post-tension anchor system.
It is still a further object of the present invention to provide an improved anchor body which is relatively inexpensive, easy to manufacture, and easy to use.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.